During the handling of samples of chemicals and other materials, it is often necessary to heat the samples to effect a desired reaction or result. The samples are often contained in vessels or tubes and, in some applications, are heated and/or cooled in a circulating water bath. The circulating water bath provides an excellent heat transfer medium in the form of water or water-like solutions. While water bath heaters perform very well, they have certain disadvantages with respect to their expense, potential cross-contamination issues, and the potential that water will escape the confines of the bath.
In other applications, the samples are heated and/or cooled in a dry bath. With a dry bath, a thermally conductive block, for example, an aluminum block, contains a plurality of blind holes into which the vessels or tubes containing the samples are inserted. Normally, an electric heater and temperature sensor are associated with the thermally conductive block. A heat control, for example, a stored program control, is utilized to provide a temperature set point to which the samples are to be heated. Different heat controls are known which control the operation of the heater as a function of the temperature set point and a temperature feedback provided by the temperature sensor. The heating cycle duration may be automatically controlled by a timer within the heat control or may be manually controlled by an operator physically removing the samples from the dry bath.
Dry baths have a significant disadvantage with respect to water baths in that the transfer of heat to and from the samples in a water bath is very efficient, and the heat transfer in dry baths is less efficient. Thus, operating cycles of a dry bath are often longer than equivalent operating cycles in a water bath. Some applications permit a temperature sensor to be mounted within the samples themselves. As can be seen in FIG. 8, in heating the samples to a selected temperature set point, for example, 60.degree., the temperature of the dry block supporting the samples may be heated to a temperature above the 60.degree. temperature set point. With such direct monitoring, the sample temperature is accurately controlled, thereby providing for an efficient and relative short operating cycle. However, directly monitoring of sample temperature increases the risk of sample contamination and the risk of the sample escaping into the environment.
Therefore, in most applications, the temperature sensor is mounted with respect to, and measures the temperature of, the heater block supporting the samples. At the beginning of a heat control cycle, the operation of the electric heater is controlled to bring the temperature of the heater block to a desired temperature set point. Upon the samples being placed in the dry bath, the temperature of the samples is different, for example, less than the temperature of the heater block, thereby cooling the heater block to a temperature less than temperature set point. The heaters in the heater block are then operated to raise the temperature of the heater block and the samples to the desired set point temperature. However, the temperature of the samples lags the temperature of the heater block; and therefore, the temperature of the samples reaches the desired temperature set point at some time after the heater block achieves that temperature. Further, the temperature of the samples normally approaches the temperature set point of the heat block asymptotically as shown in FIG. 9. That is, as the temperature of the samples approaches the temperature set point, the rate at which the samples continues to change temperature drops significantly. Thus, the time required for the samples to reach the temperature set point is extended. Since many chemical, biological or DNA reactions are dependent on the samples being at the set point for a period of time, the process is optimized if the samples reach the temperature set point as quickly as possible.
Preferably, the temperature of the sample material within the tubes should be raised quickly and consistently to the temperature set point without overshoot. However, a relatively slow heat transfer from the block to the samples within the tubes results in a relatively slow thermal response within the sample. Thus there is a need for a dry bath temperature control providing an improved heat control cycle so that the samples reach the temperature set point as quickly as possible without overshoot.